USE OF MODEL EXPERIMENTS IN SOLVING QUESTIONS OF NAVIGATION WITH SPECIAL REFERENCE TO THE ENTRANCE OF ST. ANNA BAY, CURACAO, NETHERLANDS ANTILLES

For a great many years model experiments with ships have been carried out in towing tanks in order to ascertain the reactive movements of ships under different wave and current conditions. Such experiments do not only pertain to the stability of the ships but also to forces brought to bear on the hulls of the ships. A recent development in the coastal engineering field includes the use of ship models in questions of navigation.

The reason why such experiments are undertaken is that numerous accidents, such as collisions and run-agrounds, occur when ships enter harbors and inlets* Very often such accidents are caused by adverse wave and current conditions* In the laboratory it is possible to duplicate the actual wave and current conditions and studies can be made by using models of ships oi the Influence of waves and currents on the navigation of ships approac ing or passing through the harbor* The importance of these studies is obvious* First, It is posslb!for pilots to study the problems in detail and thereby gain more expea ience about how to navigate under certain conditions, and next, reco» mendations for Improvements to the entrance of the harbor can be made, The following is a description of the conditions found In  COASTAL ENGINEERING Survey St. Anna Bay.In the foreground.theharbor entrance, on the right-hand side of the entrance Waterfort, on the left-hand side Riffort.
Entrance St. Anna Bay.The bulkhead of the bridge which runs far into the entrance channel is clearly visible.On the left top the Shell refineries .

OF ST. ANNA BAY, CURACAO, NETHERLANDS ANTILLES
The entrance to the harbor, the mouth of St. Anna Bay, lies in the center of Willemstad, the capital of Curacao.This narrow entrance is bounded by two old fortresses (Waterfort and Riffort).Over a range of 800 ft. the entrance channel narrows from 650 to 300 ft.near the portlights (measured between the drop lines of 30 ft. below low water) (Figs. 2,3,40.The underwater slopes of the channel are very steep.
At about 400 ft.inwards from the portlights there is a pontoon bridge which opens in a westerly direction, and which then has a navigable passage of 380 ft.
The mouth of the channel is bounded on either side by coral reefs.From the coast line the surface of those reefs slopes gently down to a 30 ft. depth.Then the slope grows rapidly steeper, and the surface inclines to great depths at an angle of *»-5* The fairway is not only marked by the portlights at the narrowest spot, but also by two buoys at sea.At the harbor mouth there is, in general, an easterly littoral current.This west-going current is mostly weak, but occasionally it can reach a considerable velocity for a short time.Several times velocities of k ft./sec.have been measured near the easterly buoy.
An easterly trade wind blows in Curacao.No data are available concerning the wind at the mouth of St. Anna Bay but only as to the wind on the north side of the island where the airport is situated.From a short series of comparative measurements it has been proved that in the daytime the wind at the airport has reversed itself as compared with the wind near the buoys at sea at the harbor mouth.It also has been proved that the wind velocities are greater near the airport than at the harbor mouth.During the periods of drought an average wind velocity of ib knots is normal.In contrast to the wind forces near the buoys at sea there is but little wind between the high walls of the fortresses.
The difference between the water lines at low tide and at high tide amounts to approximately 1 ft.The tidal currents in St. Anna Bay are not strong -the maximal steepness of the local tidal diagram dz appears to be 2.i0~5 m/sec.dt

DIFFICULTIES WHEN POTTING INTO THE HARBOR
When putting into the harbor ships meet with difficulties when there is a strong easterly littoral current.
These difficulties arise when the ships pass from the sea current Lnto the relatively smooth water in the harbor mouth where there is insufficient space for correction after possible sheering.
In most cases, when a maneuver has failed, the ship turns to star-»ard when passing the easterly buoy as a result of the pressure of the and Waterstaat In the Netherlands.It is remarkable that in his report Dr. Ringers points to the Influence of the wind.When coming between the fortress walls a ship comes into an area where there is but a faint current and to a certain extent, this is also the case with the wind.
Owing to a series of difficulties that arose during a highly critical period of strong currents at the harbor mouth in the middle of 1953* during which period there were many collisions, the harbor-master, who is also head of the Pilotage Service, ordered the old 3*000 ton tanker "Susane" to be stationed at about 200 ft.east of the easterly buoy to serve as a temporary breakwater.
By means of this temporary breakwater the current was diverted to a point outside the harbor.According to the pilots this improved the situation.After some weeks the tanker had to be removed because postponement would render salvage impossible.
After the installation of the temporary breakwater the Council of Curacao appointed a technical commission, whose personnel included the Harbor-master, the Director of Public Works and the Marine Superintendent of the C.P.I.M.
The advice given by the commission was practically the same as that of ir J. B. Schljf, Engineer-in-Chief of the Research Division of the Ministry of Transport and Waterstaat, who had in the meantime been appointed adviser, and who had come over from the Netherlands to Curacao in order to give advice.As has been mentioned before, the difficulties are caused by the transition of the sea current into the relatively smooth water in the harbor mouth.Therefore it is obvious that the situation would be improved by shifting this sudden change in the strengths of the currents as far outside the harbor mouth as possible, which has also been demonstrated in practice by means of the test with the tanker "Susane".Once more the Hydraulic Laboratory was charged with the investigation of the situation in the harbor mouth, as regards the currents, after a breakwater would have been built.

THE EXECUTION OF DREDGING WORK BEFORE! RIFF0RT
On the west side, the entrance channel can still be broadened considerably, which would facilitate maneuvering.

THE DJSTALIATION OF A CURRENT METER
As the velocity of the current at the harbor mouth can increase considerably within a short period, a permanently installed current meter, provided with an indicator and a recorder, was recommended.
Then the indicated strength of the current would always be visible at sea* THE BROADENING AM) DEEPENING OF THE NARROW CROSS-SECTION BETWEEN THE PORTLIGBTS In order to facilitate steering the ships it was recommended to make the narrow cross-section between the portlights broader and deeper.

THE REMOVAL OF CABLES LYING IN THE FAIRWAY
Between the two banks, near the pontoon-bridge, there are the pulling-eable of the pontoon-bridge, and several electric and telepho cables, which give much trouble if an anchor has to be used.Fig. 8. Aspects of currents in the harbor mouth after the breakws has been constructed.

THE PRESENT SITUATION
In order to make it possible to study the present situation, the model has been brought into agreement with the measurements carried out on the spot* This has been done with regard to the directions of the currents as well as with regard to the interrelations of their velocities in the various places, in so far as it was possible to reconstruct the aspect of the currents from the observation material.Of importance is the great velocity-gradient between the main current and the slowly turning eddy current.This transition is not stable and is characterized by small whirling currents between the main current and the eddy, which flow in the direction of the main current.
The result of the detailed measurements of the currents in a model of the present situation is shown in Fig. 6.The influence of the tidal current in St. Anna Bay, near the limits of the eddy current, has also been studied but proved to be imperceptible.

INVESTIGATION OF POSSIBI£ IMPROVEMENTS IN THE MODEL
As mentioned in the foregoing, an old tanker was used as a breakwater in 1953* ^n carrying out measurements with the tanker in the model, the limit of the eddy current was found to be at about the same spot as had been found in reality during the measurements carried out by means of a float (lj>0 ft.seaward from the limits of the eddy in Fig. 6).
Further, various ground-plans of the breakwater have been investigated in the model.The head of the breakwater was always placed at the edge of the reef at a depth of ko ft., as this head can hardly be placed farther away owing to the steep underwater slope.
The ground-plans shown in Fig. 7 always proved to have the limits of the eddy lying at about 180 or 200 ft.seaward from the limits of the eddy in Fig. 6.Since the ground-plan of the breakwater does not appear to exert any appreciable influence, the recommended plan has been taken as a basis, and has been subjected to a more detailed examination, a survey of which is given in Fig. 8.The currents have also been investigated at various depths in the harbor mouth.with this ground-plan of the breakwater it appears that the current over the reef head has not become appreciably stronger.Further, it appears that a secondary eddy current has been formed.The return current of the eddy is situated at about 400 ft.outside the connecting line of the portlights, and amounts to *K$ to 60$ of the main current.This is 30# to 50$ more than the return current in the present situation (Fig, 6), but a favorable circumstance is that this return current lies 200 ft.farther seaward so that there is a greater opportunity of regaining the correct eourse.The cause of the greater intensity of the return current should be sought in the fact that the main current drives the eddy on over a larger surface.(1) In the present situation the best way of putting into the harbor is to sail dead slow against the stream, parallel to the coast at about a quarter of a mile from it* Just before the leading lights are seen in a line the helm should be put a little to port, and one has to steer direct for the buoy* When the ship has come at about a ship's length from the buoy, the he2a must be put entirely to port, so that the stern is pushed against the stream* With supertankers full speed ahead is required for it* When the stern is out of the main current, the helm should be righted* (2) The presence of the breakwater does not facilitate in itself the maneuvers to put the T2 and 02 tankers into the harbor.It is important, however, that in case the maneuver fails, the tanker grounds further outside the harbor, so that the risk of damage is considerably reduced* The advantage is most marked, however, when supertankers are concerned* In the present situation full speed ahead has always been necessary, when maneuvering, in order to put into port allright.When there is a breakwater ships can also enter dead slow, which is a great advantage in view of the dangers attendant on the increase in velocity of most of these tankers* (3) The dredging away of the reef at Riffort is recommended since, in case of successful maneuvers with supertankers, there is risk of running aground on the westerly reef with the stern.With every type of tanker the advantage is that when the ship drifts off to the westerly reef during a failed maneuver, there is a greater possibility of easting anchor in time* (*0 The present leading line is ot little use when maneuvering* It is not a matter of keeping a certain course but rather of carrying out a certain maneuver at the proper moment*  In order to prevent the wave motion from influencing the results, columns of air and silicon oil have been applied between the points of pressure and the pressure gauge.In the column of silicon oil a plate has been fixed, provided with a narrow hole, which plate acts as a resistor.This resistor serves to eliminate the influence of the differences in pressure caused by the waves, Silicon oil is used because it is chemically inert, Big.11 gives a schematic outline of the operation.The measurements are transmitted electrically to the shore and recorded.Signal lamps indicate the velocity of the current at sea and are visible at sea at all times, No decision has yet been made as to the manner in which the breakwater will be constructed.Since the breakwater has to be built as far seaward as possible it win be necessary that the end of the breakwater be constructed with vertical walls.Information about waves was not available at the time of research on this paper.At the present time a wave recorder is installed near the harbor entrance to obtain basic information for design, Fig. 1 .Netherlands Antilles current on the stern, Then the ship threatens to ran aground near the green portllght or a little farther on* In some eases the tendency of running aground can be successfully cheeked by carrying out a correcting rudder and machine maneuver* Then however* the ship has a tendency to turn to the port side* which cannot be checked* The ship then runs the risk of going aground on the vest side near the Otrabanda bridge head* Finally, there are a number of cases in which the ship, either owing to excessive correction of the expected turning to starboard, or because of too little speed, turns to the port side and is in danger of running aground under Rif f ort or at the Otrabanda bridge head, or of running into that bridge head* The risk of sustaining damage is considerably increased by the presence of the pontoon-bridge and of cables lying in the fairway* In ease the pontoon-bridge should be damaged, the vehicular traffi that is using the bridge (8,000 vehicles per day) has to be diverted over a distance of about 16 kms* Pedestrians are transported by ferry< Up to now it has always been possible to repair the damage caused to tl bridge within a few weeks because spare parts and pontoons are availab! in Curacao.The difficulties presented themselves especially just before the Second World War and have continued since that time* This is due to t] greater intensity of the shipping traffic; the fact that the ships are getting bigger and bigger; and to the fact that some post-war types of tankers, which frequently put into port, steer badly* Whether a change in the staffing of the pilotage service could ex ert any influence cannot easily be traced* SUHVET OF THE ADVICE GIVEN AND THE MEASURES TAKEN IN 195^ In i°A8 the Hydraulic laboratory at Delft was charged with the ir vestigation of the possibility of shifting the current farther outsid* the mouth* A model was built on a scale of i to 1^* From the investigatioi carried out in this model it proved to be possible to shift the currei about 250 ft.seaward by building a breakwater about 350 ft* long in the sea* In view of the outlay in money involved, however, there was, some doubt as to the question whether the construction of such a breal water would serve any purpose* Consequently it was decided not to pr ceed with the building of this breakwater* In 1951 the entrance channel was broadened by means of dredging the advice of Dr* ir Ringers, ex-Engineer-in-Chief, Director of the Ministry of Transport and Works, Waterstaat, and e»-Minister of Trans IMPROVEMENTS PROPOSED BY.IR J. B. SCHIJF IN JANUARY 195*1-An illustration of the proposed improvements is shown in Fig. 5» THE C0NSTRDCTI0N OP A BREAKWATER

Further, it has
been Investigated whether there Is a possibility o weakening the return current by making an aperture In the breakwater, and by dredging a channel into the reef* This proved indeed to be possible in the model, but not in reality, as the channel would have to be dredged very accurately under cross-section, which is not feasible in practice* Besides, frequent dredging work at sea would be very expensive because there is no dredger available in Curacao which can be usec at sea* The dredging work carried out at Bif f ort appears to exert no influence on the limits of the eddy* Nor does it entail an appreciable change in the current system in the harbor south* SAILING TESTS CARRIED OUT IN THE MODEL WITH AND WITHOUT BREAKWATER A model was made on a scale of 1 to 6& of three current types are driven by an electromotor and are electrical: steered* The velocity of the rudder movement has been brought into agreement with that of the tankers* The contact between the shore an the ship consists of a composite cable which, by means of a fishing r is held in such a position that no tensile stress is exerted on the vessel* The time that is needed for the switching over from full speed ahead to full speed astern is different for each ship* If it is 1 ml ute In reality, it is 8 seconds in the model since it is possible to switch over more quickly in the model* The reaction time of the arm on the bridge, and that of the controller of the model are equal* Cc sequently one has to react 8 times more quickly in the model* Therefore the controller only suoceeds in sailing with the models by practice* As the model had been placed in the open air, the sailing tests had to be carried out when there was very little wind* Wind-screens were put up around the model* Sailing tests were carried out with ea of the models of the three types of tankers, during which tests the model was brought into agreement with the present situation in the Curacao harbor* The behavior of the model tankers and the difficulties in eontrolling these tankers appeared to correspond very well with reality, since the failing of the maneuvers led to the same consequences* Further tests were made in the situation where there is a breakwater, and where the dredging work at Eiffort has been performed* Some random trial trips of the maneuvers have been recorded in figs* 9 and 10.It was of very great importance that the harbor-waster and some other navigation experts attended the sailing with the models* After having carried out the sailing tests the following conclusions have been drawn* Fig. 9aFig.9b

Fig. 10a
Fig. 10a Maneuver QtZ Tanker With helm a little to port the tanker makes straight for the buoy.In position 3 helm completely to port* The bow threatens to turn a little to starboard* This is obviated by sailing fall speed ahead between positions 3 and k 9 for a short moment* The port screw keeps revolving dead slow ahead* In position 4-both screws again dead slow ahead* The helm is still kept entirely to port* In positions 5 and 6 helm righted* Pig* 10b Maneuver 62 Tanker Tanker correctly makes straight for the buoy* Both screws revolve dead slow ahead* In position 1 helm a little to port, in position & helm completely to port.Then, before the stern has come into smooth water the helm has already been righted, so that the bow turns to starboard and the tanker runs ashore near the green portlight.In position 5 the helm should still have been put entirely to port, only after position 5 should it have been righted slowly*